Locking assembly for a polyaxial bone anchoring device

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. patent application Ser. No. 14/290,372, filed May 29, 2014, which is a continuation of U.S. patent application Ser. No. 13/663,171, filed Oct. 29, 2012, which claims the benefit of U.S. Provisional Patent Application Ser. No. 61/552,554, filed Oct. 28, 2011, the contents of which are hereby incorporated by reference in their entirety.

BACKGROUND

Field of the Invention

The invention relates to a locking assembly for a polyaxial bone anchoring device for use in spinal or trauma surgery. The polyaxial bone anchoring device includes a bone anchoring element with a shank to be anchored in a bone and a head. The head is pivotably held in a receiving part and can be fixed at an angle relative to the receiving part by applying pressure onto the head via a pressure element. The receiving part has a recess with a substantially U-shaped cross-section for receiving a rod. Further, the locking assembly includes a first locking element that cooperates with the receiving part and is configured to fix the rod in the receiving part, and a second locking element configured to be arranged in the first locking element that is configured to fix the head via the pressure element. The first locking element has an engagement structure for a drive tool, and the second locking element is driven upon rotating the first locking element, so that the head and the rod can be fixed in a sequential manner using a tool with a single drive portion.

Description of Related Art

US 2003/0100896 A1 describes a bone anchoring device with a shank and a receiving part connected to it for connecting to a rod. The receiving part has a recess having a U-shaped cross-section for receiving the rod with two open legs and an internal thread on the open legs. A locking assembly is provided comprising a nut member with an external thread that cooperates with the internal thread of the legs and a set screw. The nut member has on one end slits for engagement with a screw tool. The shank has a spherically-shaped head that is pivotably held in the receiving part. A pressure element is provided that exerts pressure onto the head when the nut member is tightened. By tightening the set screw the rod is fixed in the receiving part. Hence, the rod and the head can be locked independently from each other.

U.S. Pat. No. 7,972,364 describes a locking assembly for securing a rod in a rod receiving part of a bone anchoring device that includes a first locking element and a second locking element. With the first locking element and the second locking element the head of the bone anchoring element and the rod can be locked independently using a tool with two drive portions.

SUMMARY

It is an object of embodiments of the invention to provide a locking assembly and a bone anchoring device with such a locking assembly that is simple to use and has advantages in certain applications.

The locking assembly according to embodiments of the invention is particularly applicable to polyaxial bone anchoring devices. It allows locking of a head of a bone anchoring element and a rod in a receiving part in a sequential manner using a single tool with a single drive portion. By this sequential locking mechanism, it is possible to first lock the head and thereafter finally lock the rod. In more detail, with embodiments of the locking assembly, first, a full locking of the head and the rod can be carried out, and thereafter, the fixation of the rod can be loosened to perform adjustments of the rod. Because only a single tool with a single drive portion is needed for performing these steps, the use of the locking assembly is more easily facilitated.

The locking principle of sequential, independent head and rod fixation according to embodiments of the invention is different from related art. The outer locking element serves for fixation of the rod and the inner locking element serves for fixation of the head. The inner locking element can transfer higher forces compared to the outer locking element. Therefore, the locking assembly can be designed to be more compact, which results in a more compact and more low profile design of the bone anchoring device.

Embodiments of the locking assembly cooperate with existing receiving parts that have an inner thread on a rod receiving portion of the receiving part. Therefore, the locking assembly according to embodiments of the invention can be used interchangeably with existing locking assemblies.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the invention will become apparent from the description of embodiments by means of the accompanying drawings. In the drawings:

<figref idref="DRAWINGS">FIG. 1</figref> shows a perspective exploded view of a bone anchoring device with a locking assembly according to an embodiment;

<figref idref="DRAWINGS">FIG. 2</figref> shows a cross-sectional view of the locking assembly of <figref idref="DRAWINGS">FIG. 1</figref>, the cross-section taken in a plane perpendicular to an axis of an inserted rod;

<figref idref="DRAWINGS">FIG. 3</figref> shows an enlarged perspective view of the locking assembly of <figref idref="DRAWINGS">FIG. 1</figref>;

<figref idref="DRAWINGS">FIG. 4</figref> shows a perspective exploded view of a modified locking assembly; and

<figref idref="DRAWINGS">FIGS. 5<i>a </i>to 5<i>c </i></figref>show steps of use of an embodiment of the locking assembly together with a polyaxial bone anchoring device.

DETAILED DESCRIPTION

A polyaxial bone anchoring device according to an embodiment of the invention is shown in <figref idref="DRAWINGS">FIG. 1</figref>. The polyaxial bone anchoring device includes an anchoring element <b>1</b> having a shank <b>2</b> with a threaded portion and a head <b>3</b>. The head <b>3</b> has a spherically-shaped outer surface portion and, on a side opposite to the shank <b>2</b>, a recess <b>4</b> for engagement with a tool. A receiving part <b>5</b> is provided for coupling the bone anchoring element <b>1</b> to a rod <b>6</b>. In the receiving part <b>5</b>, a pressure element <b>7</b> can be arranged to exert pressure onto the head <b>3</b> of the bone anchoring element <b>1</b>.

The bone anchoring device further includes a locking assembly <b>8</b> having a first locking element <b>9</b> and a second locking element <b>10</b> that can be coupled to the first locking element <b>9</b>.

As shown in <figref idref="DRAWINGS">FIG. 1</figref>, to fix the rod <b>6</b> and lock the head <b>3</b>, a tool <b>100</b> with a single drive portion may be provided that engages the first locking element <b>9</b>.

Referring to <figref idref="DRAWINGS">FIG. 1</figref> and <figref idref="DRAWINGS">FIGS. 5<i>a </i>to 5<i>c</i></figref>, the receiving part <b>5</b> has a top end <b>51</b> and a bottom end <b>52</b>, and includes portions having a substantially cylindrical construction, with a longitudinal axis C extending through the two ends <b>51</b>, <b>52</b>. Coaxial with the longitudinal axis C, a bore <b>53</b> is provided extending from the top end <b>51</b> to a predetermined distance from the bottom end <b>52</b>. At the bottom end <b>52</b>, an opening <b>54</b> is provided, a diameter of which is smaller than a diameter of the bore <b>53</b>. The coaxial bore <b>53</b> tapers towards the opening <b>54</b>. In the embodiment shown, the taper is in the form of a spherically-shaped section <b>55</b>. However, the section <b>55</b> can have any other shape such as, for example, a conical shape, that facilitates the function of the head <b>3</b> being pivotably held in the receiving part <b>5</b>, for example, similar to a ball and socket joint.

The receiving part <b>5</b> further has a U-shaped recess <b>56</b> extending from the top end <b>51</b> in the direction of the bottom end <b>52</b>. By means of the U-shaped recess <b>56</b>, two free legs <b>57</b>, <b>58</b> are formed. The recess <b>56</b> is open towards the top end <b>51</b> and defines a channel for receiving the rod <b>6</b>. Adjacent to the top end <b>51</b>, an internal thread <b>59</b> is provided at inner surfaces of the legs <b>57</b>, <b>58</b>. In the embodiment shown, the internal thread <b>59</b> is a flat thread having substantially horizontal upper and lower thread flanks. However, any other thread form can be used for the internal thread <b>59</b>, but, a thread form that reduces or eliminates splaying of the legs may be preferable, such as a flat thread or a negative angle thread. The internal thread <b>59</b> has a first thread pitch.

The pressure element <b>7</b> has a substantially cylindrical construction, with an outer diameter sized so as to allow the pressure element <b>7</b> to be introduced into the bore <b>53</b> of the receiving part <b>5</b> and to be moved therein in an axial direction. The pressure element <b>7</b> has a top end <b>71</b> and an opposite bottom end <b>72</b> and a longitudinal axis extending through the two ends, where the axis is coaxial with the longitudinal axis C of the receiving part <b>5</b> when the pressure element <b>7</b> is inserted therein. The pressure element <b>7</b> is arranged in the receiving part <b>5</b> such that its top end <b>71</b> is oriented towards the top end <b>51</b> of the receiving part and the bottom end <b>72</b> is oriented towards the bottom end <b>52</b> of the receiving part. At its top end <b>71</b>, the pressure element has a cylindrically-shaped recess <b>73</b> that is configured to receive the rod <b>6</b>. When the rod <b>6</b> rests in the recess <b>73</b>, the top end <b>71</b> of the pressure element <b>7</b> does not extend past the surface of the rod <b>6</b> in a direction of the top end <b>51</b> of the receiving part <b>5</b>. On its lower side, the pressure element <b>7</b> has a spherically-shaped recess configured to cooperate with the spherical outer surface portion of the head <b>3</b>. Furthermore, a coaxial through-hole <b>75</b> is provided in the pressure element <b>7</b> that allows access to the recess <b>4</b> of the head <b>3</b>, for example, by a tool, when the bone anchoring element <b>1</b> and the pressure element <b>7</b> are mounted to the receiving part <b>5</b>.

As can be seen in particular in <figref idref="DRAWINGS">FIGS. 2 and 3</figref>, the first locking element <b>9</b> of the locking assembly <b>8</b> is a screw, for example, a set screw, with a top end <b>91</b> and a bottom end <b>92</b>. The first locking element <b>9</b> forms an outer locking element in the assembly <b>8</b>. Between the top end <b>91</b> and the bottom end <b>92</b>, an outer surface portion with an external thread <b>93</b> is provided that cooperates with the internal thread <b>59</b> of the receiving part <b>5</b>. In the embodiment shown, the external thread <b>93</b> is a flat thread having a first thread pitch corresponding to the first thread pitch of the internal thread <b>59</b>. A coaxial through-hole extends from the top end <b>91</b> to the bottom end <b>92</b>, the through-hole has a portion with an internal thread <b>94</b> adjacent to the bottom end <b>92</b>. The internal thread <b>94</b> has a second thread pitch that is smaller than the first thread pitch of the outer thread <b>93</b>. The internal thread <b>94</b> is, in this embodiment, a flat thread. Thus, the thread <b>94</b> is capable of transferring high loads. The use of a flat thread as the internal thread <b>94</b> allows the first locking element to have a smaller size that contributes to a low profile bone anchoring device. Adjacent to the top end <b>91</b>, a coaxial cylindrical recess <b>95</b> with an engagement structure <b>96</b> is provided that is engageable by the tool <b>100</b>. The engagement structure <b>96</b> can be, for example, in the form of longitudinal grooves extending parallel to a central axis of the first locking element <b>9</b>, like a torx-shape. The second thread pitch of the internal thread <b>94</b> can be, for example, three-quarters of the first thread pitch of the external thread <b>93</b>. A relationship between the second thread pitch and the first thread pitch is selected such that a predefined advancement path for the first and second locking elements <b>9</b>, <b>10</b>, within the receiving part <b>5</b> is achieved.

The second locking element <b>10</b> forms an inner locking element of the assembly <b>8</b>. The second locking element <b>10</b> has a top end <b>101</b> and a bottom end <b>102</b>, a first portion <b>103</b> adjacent to the top end <b>101</b> and a second portion. <b>104</b> between the first portion <b>103</b> and the bottom end <b>102</b>. The first portion <b>103</b> is cylindrical with an outer surface portion having an external thread <b>103</b><i>a </i>that cooperates with the internal thread <b>94</b> of the first locking element <b>9</b>. In the embodiment shown in <figref idref="DRAWINGS">FIGS. 1 to 3</figref>, the external thread <b>103</b><i>a </i>is a flat thread. The thread pitch of the external thread <b>103</b><i>a </i>corresponds to the second thread pitch. An axial length of the first portion <b>103</b> is such that when the second locking element <b>10</b> is fully engaged within the through-hole of the first locking element <b>9</b>, the top end <b>101</b> of the second locking element <b>10</b> may be substantially flush with the top end <b>91</b> of the first locking element <b>9</b>, as can be seen in particular in <figref idref="DRAWINGS">FIG. 5<i>c</i></figref>. In other embodiments, the second locking element <b>10</b> may be farther above or below the top end <b>91</b> of the first locking element <b>9</b> when the first and second locking elements <b>9</b> and <b>10</b> are fully engaged.

The second portion <b>104</b> has a saddle-like shape with two outward and downward projecting saddle portions <b>104</b><i>a</i>, <b>104</b><i>b </i>that define a substantially U-shaped recess therebetween. The length of the saddle portions <b>104</b><i>a</i>, <b>104</b><i>b </i>are such that when the bottom end <b>102</b> is in contact with the top end <b>71</b> of the pressure element <b>7</b> and the rod <b>6</b> is placed between the saddle portions <b>104</b><i>a</i>, <b>104</b><i>b </i>and the pressure element <b>7</b>, there is a gap <b>107</b> between a top of the saddle-shaped region and the rod <b>6</b>, as shown in <figref idref="DRAWINGS">FIGS. 5<i>a </i></figref>and <b>5</b><i>b. </i>

Referring to <figref idref="DRAWINGS">FIG. 3</figref>, on either side of each saddle portion <b>104</b><i>a</i>, <b>104</b><i>b</i>, two outwardly projecting edges <b>105</b> or projections are provided. More specifically, four edges <b>105</b> that project in a direction of the rod axis when the rod is inserted are provided. The edges <b>105</b> are located at a distance from the bottom end <b>102</b> and project outward to such an extent that when the second locking element <b>10</b> is mounted to the first locking element <b>9</b> and the locking assembly <b>8</b> is inserted into the receiving part <b>5</b>, the edges <b>105</b> prevent rotation of the second locking element <b>10</b> with respect to the receiving part <b>5</b>. Rotation of the second locking element <b>10</b> may be prevented because the edges <b>105</b> cause portions of outermost parts of the saddle portion <b>104</b> to form, for example, edges of a rectangle, which may abut against, for example, the internal thread <b>59</b>, sides of the recess <b>56</b>, or other portions of the bore <b>53</b> of the receiving part <b>5</b> when the second locking element <b>10</b> is in the receiving part <b>5</b>. An outer diameter of lower portions of the saddle portions <b>104</b><i>a</i>, <b>104</b><i>b </i>adjacent the bottom end <b>102</b> is smaller than an inner diameter of the coaxial bore <b>53</b>, so that the saddle portions <b>104</b><i>a</i>, <b>104</b><i>b </i>can extend into the coaxial bore <b>53</b>, as can be seen in <figref idref="DRAWINGS">FIGS. 5<i>a </i>to 5<i>c</i></figref>. Further, a small engagement portion <b>108</b> can be provided in the second locking element <b>10</b>.

The locking assembly <b>8</b> may be preassembled in such a manner that the second locking element <b>10</b> is arranged with its first portion <b>103</b> within the first locking element <b>9</b>, while the second portion <b>104</b>, as well as a section of the first portion <b>103</b>, projects outward from the bottom end <b>92</b> of the first locking element <b>9</b>, as shown in <figref idref="DRAWINGS">FIGS. 2 and 3</figref>. The second locking element <b>10</b> may be fixed in a preliminary manner to the first locking element at a predetermined breaking point <b>106</b>, which may be for example, a welding point that is provided near the bottom end <b>92</b> at a contact area between the external thread <b>103</b><i>a </i>of the second locking element <b>10</b> and the internal thread <b>94</b> of the first locking element <b>9</b>. The welding point <b>106</b> has a fixation strength such that it can be broken by rotating the first locking element <b>9</b> with respect to the second locking element <b>10</b> using, for example, a handheld tool <b>100</b>. By the preliminary fixation of the second locking element <b>10</b> to the first locking element <b>9</b>, a predetermined height relationship or relative position between the second locking element <b>10</b> and the first locking element <b>9</b> can be provided and maintained before inserting the locking assembly <b>8</b> into the receiving part <b>5</b>.

<figref idref="DRAWINGS">FIG. 4</figref> shows a modified embodiment of the locking assembly. The locking assembly <b>8</b>′ differs from the locking assembly <b>8</b> described above in that the saddle portion <b>104</b> has at a distance from the bottom end <b>102</b> four tabs <b>105</b><i>a</i>′ that prevent rotation when inserting the locking assembly <b>8</b>′ into the receiving part <b>5</b>. The tabs <b>105</b><i>a</i>′ have triangular cross sections. In addition, instead of a flat thread, a metric thread <b>103</b><i>a</i>′ is used as the external thread for second locking element <b>10</b>′. Other parts are the same or similar to the first embodiment.

The parts of the bone anchoring device are made of a biocompatible material, for example, of a biocompatible metal or metal alloy, such as titanium, stainless steel, of nickel titanium alloys, such as Nitinol, or of a biocompatible plastic material, such as PEEK (polyetheretherketone). The parts can be made of the same or of different materials.

A function or operation of the locking assembly <b>8</b> may be as follows. The locking assembly <b>8</b> is preassembled as shown in <figref idref="DRAWINGS">FIG. 2</figref>, wherein the top end <b>101</b> of the second locking element <b>10</b> is located at a distance d from the top end <b>91</b> of the first locking element <b>9</b> and is fixed with the welding point <b>106</b> in a preliminary manner. In use, when the locking assembly <b>8</b> is inserted into the receiving part <b>5</b> and the saddle <b>104</b> engages the rod <b>6</b>, the second locking element <b>10</b> is prevented from rotation, and a clockwise rotation of the first locking element <b>9</b> allows the first locking element <b>9</b> to travel downward relative to the receiving part <b>5</b>. Meanwhile, the second locking element <b>10</b> travels upward relative to the first locking element <b>9</b>.

In use, the bone anchoring device is operated as follows. First, at least two bone anchoring devices including the bone anchoring element <b>1</b>, the receiving part <b>5</b>, and the pressure element <b>7</b>, which may be preassembled are screwed into a bone or a vertebra. Thereafter, the rod <b>6</b> is inserted into the U-shaped recess <b>56</b> of the receiving part <b>5</b>. Then, the locking assembly <b>8</b> that is preassembled as described above can be inserted between the legs <b>57</b>, <b>58</b>, until the external thread <b>93</b> of the first locking element <b>9</b> engages the internal thread <b>59</b> provided on the legs <b>57</b>, <b>58</b>. Torque applied to the first locking element <b>9</b> with the tool <b>100</b> will break the welding point <b>106</b> between the first locking element <b>9</b> and the second locking element <b>10</b>, allowing the second locking element <b>10</b> to travel vertically relative to the first locking element <b>9</b>. An initial position is shown in <figref idref="DRAWINGS">FIG. 5<i>a</i></figref>. The first locking element <b>9</b> has not moved a full turn. In this position, the bottom end <b>102</b> of the second locking element <b>10</b> is not yet in contact with the top end <b>71</b> of the pressure element <b>7</b>. Furthermore, there is a gap <b>107</b> between an upper surface of the rod <b>6</b> and an upper region of the saddle portion <b>104</b>.

Then, as shown in <figref idref="DRAWINGS">FIG. 5<i>b</i></figref>, after several turns of the first locking element <b>9</b>, the first locking element <b>9</b> has travelled downwards. The first locking element <b>9</b> is still not in contact with the rod <b>6</b>. Meanwhile, the second locking element <b>10</b> has travelled downwards a smaller distance than the first locking element <b>9</b> (e.g., due to the difference between the first and second pitches) and is now in contact with its saddle portions <b>104</b>, <b>104</b><i>b </i>pressing onto the top end <b>71</b> of the pressure element <b>7</b>. By means of this, the pressure element <b>7</b> exerts pressure onto the head <b>3</b> that clamps the head <b>3</b> by friction. Therefore, the bone anchoring element <b>1</b> can be maintained at a particular angular position with respect to the receiving part <b>5</b>.

As can be seen in <figref idref="DRAWINGS">FIG. 5<i>c</i></figref>, by further rotating the first locking element <b>9</b>, the additional torque applied to the first locking element <b>9</b> causes the first locking element <b>9</b> to press onto the rod <b>6</b> and locks the full assembly of the bone anchoring device, that is, the head <b>3</b> and the rod <b>6</b> relative to the receiving part <b>5</b>.

The travel paths of the locking elements <b>9</b>, <b>10</b> can be determined as follows: &lt;br /&gt;First locking element absolute travel path=number of turns*first pitch&lt;br /&gt;Second locking element absolute travel path=number of turns*(first pitch−second pitch)

If the first pitch is 1 mm and the second pitch is 0.75 mm, and the number of turns is 1, then in one tightening turn, the first locking element will move 1 mm downwards and the second locking element will move 0.25 mm downwards.

After full locking of the head <b>3</b> and the rod <b>6</b>, as shown in <figref idref="DRAWINGS">FIG. 5<i>c</i></figref>, the fixation of the rod <b>6</b> may be loosened by turning back the first locking element <b>9</b>, to allow adjustments of the rod <b>6</b> while the head <b>3</b> remains locked. Hence, in a sequential procedure, first, the whole assembly is locked, and then, the position of the rod <b>6</b> can be adjusted using only one tool with a single drive portion.

Various modifications of the embodiments described are also conceivable. For the polyaxial bone anchoring device, any known polyaxial bone anchoring device can be used that includes a bone anchoring element pivotably received and a rod received in a receiving part. For a bone anchoring element, any known bone anchors, such as screws, nails, hooks, among others, can be used. In addition, for example, the pressure element and/or the second portion of the second locking element may be shaped differently. For example, the pressure element can be designed with a flat upper surface that contacts the rod. In such a case, the saddle portions of the second locking element may be longer to contact the pressure element without contacting the rod at its highest surface point. Alternatively, for example, the recess where the rod is received in the pressure element can be deeper, and can have such a depth that the top end <b>71</b> of the pressure element extends above the top surface of the rod. In such a case, the saddle portions of the second locking element can be shorter. Meanwhile, the second portion of the second locking element may be rotatably coupled to the first portion of the second locking element. The internal thread of the first locking element and the external thread of the first locking element may run in opposite directions. Various other modifications may also be implemented.

While the present invention has been described in connection with certain exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but is instead intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims, and equivalents thereof.